Atrial fibrillation and ventricular tachyarrhythmias occurring in patients with structurally abnormal hearts are the most important arrhythmias in contemporary cardiology. They represent the most frequently encountered tachycardias, account for the most morbidity and mortality, and, despite much progress, and remain therapeutic challenges. Invasive studies of the electrical activity of the heart (electrophysiologic study) are often used in the diagnosis and therapy of arrhythmias, and many arrhythmias can be cured by selective destruction of critical electrical pathways with radiofrequency (RF) catheter ablation. Attempts at applying ablation to atrial fibrillation and ventricular tachycardia have been made. Success has been limited, however, by the long time duration of procedures, resulting from the difficulty of creating continuous linear lesions in a setting where areas of ablated myocardium cannot be directly visualized. Continuous linear lesions, without gaps, can block critical arrhythmogenic circuits and reduce the amount of electrically contiguous arrhythmogenic substrate, thereby eliminating arrhythmias. We hypothesize that magnetic resonance imaging (MRI), with MRI-compatible diagnostic and therapeutic systems; can allow electrophysiology studies and catheter ablation to be performed without x-ray radiation. We also hypothesize that this technology will provide the ability to visualize ablation lesions, which should greatly simplify production of continuous linear lesions, and should improve the effectiveness of ablation procedures in general. In addition to electrophysiology, these methods may be applicable to guiding other diagnostic and therapeutic techniques. In Phase I, we will complete a prototype steerable ablation catheter that will allow us to target any area of the endocardial surface of the heart. We will also develop integral filters for protecting the catheters from excessive heating during MR imaging. We will test the prototype catheters in animals to show that electrophysiology studies can be done under MR guidance alone, that lesions can be produced and imaged, that linear lesions can be produced, and that MRI has sufficient resolution to allow detection of significant gaps in the lesions. In Phase II, we will develop, test, and prepare for manufacturing and marketing, a clinical-grade version of the ablation system, and apply for FDA approval for testing the technology.